18" Wheels on Cayman S
Discussion
I recently bought some 18" wheels for my 981 Cayman S (which runs 19" wheels), the intention being to fit winter tyres. However the wheels came off a car with only nominal mileage, which had been fitted with different wheels - as seems pretty common. Goodyear Eagles were fitted and I was interested to compare them with my Pzeros before removing them and fitted winter tyres. I didn't want to put significant miles on the tyres so my tests were very limited, and only in the dry - but FWIW (and given that both tyre brand and wheel size are changed, not that scientific) this is what I found.
on 18s..
1) The steering was noticeably lighter, had more feel and was less "rubbery"
2) Understeer was much more noticeable
3) The ride - particularly at the front - was more "rounded"
4) Tyre roar from rear tyres was significantly reduced
5) Traction was worse and traction control intervened on occasion (very rare in normal driving on the Pirellis)
6) The car generally felt "loose" - on the Pirellis the car has a terrifically "planted" feel, which is both confidence inspiring and, at times, frustrating.
Don't read too much into the above - I would have needed far more miles, in differing weather conditions and push the tyres harder than I was prepared to (I want to sell the tyres as near-new).
Anyone else with similar experience?
on 18s..
1) The steering was noticeably lighter, had more feel and was less "rubbery"
2) Understeer was much more noticeable
3) The ride - particularly at the front - was more "rounded"
4) Tyre roar from rear tyres was significantly reduced
5) Traction was worse and traction control intervened on occasion (very rare in normal driving on the Pirellis)
6) The car generally felt "loose" - on the Pirellis the car has a terrifically "planted" feel, which is both confidence inspiring and, at times, frustrating.
Don't read too much into the above - I would have needed far more miles, in differing weather conditions and push the tyres harder than I was prepared to (I want to sell the tyres as near-new).
Anyone else with similar experience?
anonymous said:
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As I said the comparisons cannot be regarded as genuinely scientific, and it's impossible to genuinely separate the effects of tyre brand, wheel size and (in the case of the rears) rim width. That said the results - such as they are - wouldn't make me inclined to go either 18" or Goodyear. The big plus (steering feel) and the (predictable) improvements in ride and road noise are (for me) completely nullified by the increased understeer and the nature of the loss of rear grip. I would welcome a "loose" rear which was "playful" , but what I got was one which was not really predictable or progressive - not in a dangerous way, but simply not pleasant. I actually stopped to check if I had a flat or a tyre had lost pressure, it was that odd. anonymous said:
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Don't think there is anything wrong with the car - 19" back on and everything back to normal - same roads every time. If there is anything wrong it's the tyres - though, as you say, I expected more progressive handling not less. Not keen on understeer, aiming for less not more - a modicum is safe and reassuring, but not the degree I have with 18". Trail braking on the track is fine, on the roads - at least the ones I frequent - there is rarely enough visibility to enter a corner fast enough to sensibly trail the brakes. On 19" its perfectly possible to adjust the line with the throttle (which is what I want) - easy enough in the wet and not that difficult in the dry but you are going so damn fast to get to that point.
I think for my next step I'll take the popular Porsche approach and fit an X73 rear bar (or an adjustable one) - I have PASM (in sport mode for all the tests) . There is only a limited amount I would be prepared to do with geometry while the car is under warranty. The car will never be tracked so I'm not prepared to get even vaguely aggressive with front camber in any case. The big advantage (for me) with an adjustable rear bar is that it is very cheap, a trivial DIY fit, and can be played with at will, or reverted to standard, without having to go to a garage with the necessary equipment at vastly higher cost. It's bound to reduce understeer (which is what I want) - but we'll have to see how much negative impact it has on traction out of bends.
Edited by bcr5784 on Tuesday 26th May 09:52
anonymous said:
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Nonsense. You can only do that if you can see the exit of the bend before you commit to it. You have to brake quite hard to neutralise it - but if you are regularly needing to brake that hard to the apex of a BLIND bend you are going to have an accident - it might be the guy coming the other way who is at fault but it may be you that gets hurt. There are parts of the country where you have enough vision to do that - but not on the roads I drive on regularly - which is what I said in my post. On blind bends you have to adopt a slow in fast out approach. You have changed your car to behave the way you want it - I'd like to change mine the way I want it, which is clearly different.
Whether or not trail braking is desirable has nothing to do with the speed you are approaching or taking a corner. It is about the timing of braking, not the amount. Indeed, gentle but late braking is, in my experience, quite effect at tucking the nose into the corner even at very sedate speeds.
anonymous said:
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You are missing the point. If I go in to a bend (relatively) slowly but still with a modicum of understeer, as soon as I can see round the bend I want to accelerate - but if I do I create more understeer UNLESS I boot it hard. But if I trail brake under those circumstances to bring the back round, I actually go slower round the bend than I need to - or I commit to higher corner entry speed than I deem sensible. What I want to do is to go in on a trailing or neutral throttle until I can see and then go out using power to balance the car just enough to wind the lock off - AS I COULD EASILY DO in a Caterham. Porsche do that in two ways - a stiffer rear anti-roll bar and Torque Vectoring (which is going to be far more effective than normal trail braking because only acts on the rear). The very last thing I want to do it to create MORE understeer and then, to correct it, have to adopt a style that is, for most of the time on the roads I drive, not appropriate.
anonymous said:
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If you want to trail brake to correct what I regard as a weakness in the chassis, and you think you can safely, then fine. I'd rather sort the chassis balance out. But can we get back to the topic. Has anyone actually done any other similar sorts of tests and come to any conclusions?
anonymous said:
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Nop - I'll make rear more mobile by other means. An Elise is more playful, so there is no reason a Cayman can't be. And just for your information, on (proper) GT3 race cars (not Cup cars - at least pre 991, where the regs didn't allow it) they get rid of most of the understeer even though professional drivers can trail break around it - because they regard it as a weakness too.
As usual? Moose and I do not agree. Frankly the concept of exacerbating an existing weakness (understeer), to derive pleasure from circumventing it, is a bizarre concept. Indeed making a Cayman more like a 911 - when the Cayman is generally considered superior as is it is, is similarly odd. Understeer on the 911 was designed in to limit the number of drivers who went tail first into the hedge due to lift off oversteer in days of yore (when Porsche had semi trailing arm rear suspension, not quite the worst form of IRS, but close). Some seem to want to elevate driving around the understeer to an art form (rather than a necessity under the circumstances). It's an old fashioned skill, which clearly some derive pleasure from, a bit like double declutching - and good luck to them. Odd that the young want to look back and the old forward (perhaps because the old have been there and regard it as overrated).
But can we get back to the original topic - as I requested - or is moose going to hijack it yet again. Has anyone done back to back tests on tyres with some helpful analysis of their findings? My concern, as I keep on saying, is that because Porsche insist on their own spec tyres it's difficult to make sensible choices when it comes to replacement. Simplistic statements that a) is terrific and b) crap (as we tend to get) is hopeless - we all have different priorities. Wet/dry/grip/progression/wear/steering feel/steering weight/braking/cornering/noise - but not necessarily in that order are what we need to know - as well as the spec of the car on which they are tested.
But can we get back to the original topic - as I requested - or is moose going to hijack it yet again. Has anyone done back to back tests on tyres with some helpful analysis of their findings? My concern, as I keep on saying, is that because Porsche insist on their own spec tyres it's difficult to make sensible choices when it comes to replacement. Simplistic statements that a) is terrific and b) crap (as we tend to get) is hopeless - we all have different priorities. Wet/dry/grip/progression/wear/steering feel/steering weight/braking/cornering/noise - but not necessarily in that order are what we need to know - as well as the spec of the car on which they are tested.
Edited by bcr5784 on Tuesday 26th May 20:11
anonymous said:
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Not sure that's 100 % right, cmoose. Most car manufacturers do build in a little 'safety understeer', and Porsche could always have used wider front tyres to counteract the tendency to push wide.I expect that Porsche left the understeer (resulting fundamentally from a light front end) as it was because it made the 911 safer for most drivers.
anonymous said:
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I think you need to read Milliken and Milliken - oversteer is the natural tendency with an engine in the back with high(ish) cornering speeds. That is why you put big wheels on the back, a stiff front anti roll bar/high roll centre on the front, low on the back etc etc to counteract that tendency . The result then is initial understeer which only neutralises at high cornering speeds. Understeer is a result of the design you put in place to mitigate/overcome the natural oversteer tendency.You do the opposite on a front wheel drive car to limit the natural tendency to understeer.
But can we get back to the topic rather than have me teach you the basics of chassis design.
bcr5784 said:
anonymous said:
[redacted]
I think you need to read Milliken and Milliken - oversteer is the natural tendency with an engine in the back with high(ish) cornering speeds. That is why you put big wheels on the back, a stiff front anti roll bar/high roll centre on the front, low on the back etc etc to counteract that tendency . The result then is initial understeer which only neutralises at high cornering speeds. Understeer is a result of the design you put in place to mitigate/overcome the natural oversteer tendency.You do the opposite on a front wheel drive car to limit the natural tendency to understeer.
But can we get back to the topic rather than have me teach you the basics of chassis design.
anonymous said:
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Sorry moose if you want a fuller explanation with tyre load curves, slip angles, weight transfer, how AR bars work and practical examples of how other rear engined examples counter oversteer I could give it. But there is little point in trying to explain something to someone who is determined not to listen or understand. As the expression goes - there's none so deaf as those that don't want to hear.I don't. From my position of near-total ignorance, I can see that understeer might result from the lack of weight over the front wheels (except that would be resolved by deceleration), but I can also see that a rear-engine car would have a pendulum effect once the front tyres bite, resulting in oversteer.
The fact that Porsches have always had fat back tyres might also be to do with traction as much as grip.
The fact that Porsches have always had fat back tyres might also be to do with traction as much as grip.
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